Transmission regulation



JIIEy 3, 1928.

H. T. FRHS Filed Nov. 29, 1924 DETECTED CURRENT Q d JMPRESSED SIGNALE.M.F. X

//7V/7/0['-' Ham/d 27-7775 meme July 3, 1928.

UNITE-D STATUES r-PATENT OFFICE.

HARALD T. FRIIS, OF -RED BANK, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRICCOMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TRANSMISSION REGULATION.

Application filed November 29, 1924. Serial No. 752,891.

' This invention relates to radio receiving systems and moreparticularly to methods of and means for controlling the intensity ofreceived signals at a uniform level.

One object of the invention is to improve the constancy of the over-alltransmission equivalent in radio communication systems.

It is well known that radio signaling waves are subject to extremelyvariable attenuation and that the strength of the received signals mayvary so greatly as to make it difiicult and even impossible attimes toobtain satisfactory communication. These variations may be due to theregular changes between day and night conditions or they may be the muchmore rapid changes which are commonly known as fading effects. Slowchanges of the former kind may be compensated by manual adjustment 2 ofthe sensitivity of the receiving system, but compensation of the muchmore rapid fading efi'ects requires some form of automatic adjustment.

To maintain a uniform transmission 2 equivalent in a radio system it isnecessary that any change in the attenuation of the medium beimmediately offset by an opposite change in the gain characteristic ofthe receiving system. Methods of securing this result have been proposedby Espenschied and Bown in U. S. Patent No. 1,447,- 773, issued March 6,1923, and by Aifel in U. S. Patent No. 1,468,687, issued September 25,1923.

The methods disclosed in these patents consist in radiating along withthe signal wave a pilot wave of constant amplitude, and employing at thereceiving station currents corresponding to this wave to control thesensitivity of the receiving system. The

control apparatus, including mechanical elements, is so arranged thatifthe received pilot wave diminishes in intensity the sensitivity of thereceiving system is increased by a corresponding amount and vice versa.The pilot wave, being free from signal variations, is modified only bythe attenuation of the transmission medium and therefore the control ofthe receiver gain is responsive only to changes in the attenuation ofthe medium. In most communicat1on systems the signal is transmittedas amodulated carrier wave, which may be considered as a wave ofsinglefrequency and varying amplitude or alternatively as a group ofwaves including a constant wave of the carrier frequency and side wavesrepresenting the signal. It is therefore unnecessary to provide aspecial pilot wave as the carrier wave, when separated from the sidewaves, is satisfactory for the purpose. A transmitting system suitablefor this method of transmission is described in U. S. Patent No.1,442,147, January 16, 1923, to

Heising.

By the present invention an improved means is provided for controllingthe sensitivity of the receiving system in accordance with the intensityof the received carrier or pilot Wave. The control is effected withoutthe intervention of any mechanical device and may be made to respondaccurately to and to compensate changes in the transmitting mediumhaving periods of less than one fiftieth of a second.

In the detailed description which follows the invention will bedescribed as applied to radio telephony for the reason that the need isgreatest in that field. It is to be understood, however, that it may beapplied equally well to any type of carrier wave system regardless ofthe nature of the medium of transmission.

, Fig. 1 of the drawings shows a receiving system in which the inventionis embodied; Fig. 2 shows a special characteristic curve I f. anamplifier which is adapted to illustrate the operation of the invention;and

Fig. 3 is the operating characteristic curve of the receiving system ofFig. 1.

The receivingsystem of Fig. l is of the double detection orsuper-heterodyne type in which an intermediate frequency carrier wave isproduced by demodulation of the incoming wave with an auxiliary wavelocally generated. The incoming waves are received by the loop antenna 1which may be tuned to resonance by means of condenser 10. The auxiliarywave is supplied from a local source 2 and is impressed upon the 1.? Twostages of high frequenc waves being amplified thereby. The first stageof detection occurs in the space dis charge demodulator 5 from theoutput of which the demodulation products are delivered through atransformer 12 to the band-pass filter 6.

The function of filter 6 is to separate out and transmit only theintermedia'tefrequency waves corresponding to the difference between theincoming wave frequencies and the auxiliary wave fre uency. It ispreferable that the intermediate frequency waves should occupy a baridcentered in the neighborhood of 50,000 cycles per second, the width ofthe band being made sufficient to include the intermediate frequencycarrier and all side frequencies necessary for the proper reproductionof the signal. The filter may be constructed in accordance with themethods and formulae disclosed in U. S. Patent No. 1,227,113, issued May22, 1917, to G. A. Campbell. 2

The selected intermediate. frequency band isamplified in theamplifier-detector 7, in which the final step of detection to produce.

the low frequency signal currents also takes place. The low frequencycurrents are delivered to an amplifier 8 and the amplified current issupplied to the signal reproducer 9, which is illustrated 'as a loudspeaking' telephone.

The amplifier-detector 7 and the amplifier 8 are indicated only inconventionalform, the particular typev and arrangement used beingunimportant, since any well known devices adapted to perform the desiredfunctions may be used.

The local auxiliary wave source 2 should be adjustable so thatthe'frequ'ency of the intermediate carrier wave and the associated Iside frequencies may be within the range 7 current.

selected by the filter 6. A suitable form of auxiliary source is thespace discharge oscillator disclosed in U. S. Patent No.1,356,763,.issued October 26, '1920to R. V. L. Hartley. The controlcurrent by which the sensitivity of the receiver is regulated isselected from the output of the amplifier-detector 7 by the resonantcircuit 13, 14, connected in shunt to its output circuit, which is tuned.to. the frequency of the intermediate carrier wavel. Its voltage afterbeing amplified in the space discharge amplifier l5 is impressed uponthe space discharge detector 16 in the output of'which it produces asteady direct The control electrode 26 of the detector is so polarizedby the battery 27 that little or 'no space current fiows normally;consequently in accordance with the well known characteristics of spacedischarge detectors, an alternating E. M. F. impressed upon the controlelectrode produces a direct current in the space path which increases asthe impressed E. M. F. increases.

. The output circuit of the detector 16 includes the space dischargepath between the anode 29. and the cathode 25, a separate space currentsource 17 and a fixed resistance 18. The cathode and one terminal of theresistance 18 are connected together at ground potential. Choke coil 19and condenser 20 serve to prevent the flow in resistance 18 of anyalternating current corresponding to the impressed intermediatefrequency wave.

For reasons that will be explained later, the high frequency amplifiers3 and 4 are provided with balancing capacities 21 to neutralize thecoupling between the output and the input circuits of the amplifiers dueto the internal electrode capacities.

In the first high frequency amplifier, the cathode is connected througha relatively large'capacity 22 to the mid-point of the receiving loop,the potentials of the end terminals of which are therefore balanced withrespect to the cathode and are of opposite sign. One terminal of theloop is- .connected to.the input or control electrode 23 0f theamplifier and the other is connected through the balancing condenser 21to the anode 24. The capacity of the condenser 21 is substantially equalto the direct capacity between the electrodes 23 and 24. By this means,any current that might be transmitted directly from the input to theoutput of the amplifier through the elec-' trode capacity is neutralizedby an equal and opposite current transmitted through the balancingcondenser.

The circuit of the second stage is similar and operates in ,a similarmanner. The system of neutralization described has been disclosed in.U.S. Patent No. 1,334,118, issued March 16, 1920 to C. WV. Rice.

The control of the sensitivity of the system is secured by causing therectified out-- put current of the detector 16 to modify the controlelectrode potentials of the amplifiers 3 and 4. The cathodes of theseamplifiers are connected to groundsand the control electrodesare-connected to the ungrounded terminal of'resistance 18, whereby theyare caused to assume the potential of this point.

This potential is determined by the current upon the amplifier controlelectrodes. An

I amplifier space current-s. ,This is equivalent to increasing theinternal resistance of the amplifiers and is effectual in reducing theirover-all amplification. It follows then that an increase in theintensity of the incoming wave, by producing an intermediate frequencycarrier wave of larger amplitude, causes an increased current to flow inthe resistance 18, and thereby reduces the sensi tivity of the receivingsystem.

The relationship between the space path impedances of the amplifiers 3and 4 and the polarizing potentials of their control electrodes is shownqualitatively by the curve in Fig. 2. The ordinates are proportional tothe space path impedance; the, abscissae are proportional to the steadycurrent I flowing in the resistance 18, and therefore to the polarizingpotentials applied to the control electrodes of the amplifier. Thevertical dotted line JJ' corresponds to the potential that is justsufiicient to reduce the space current to zero, the space path impedancebecoming infinite under this condition.

The detected current I cannotexceed the value corresponding to theabscissa OJ; since it requires for its production that some part of theincoming wave be repeated by the amplifiers and this cannot take placeif the amplifiers are blocked. A continually increasing intensity of theincoming wave may cause the detected current to ap-' proach indefinitelyclose to the limiting value, but it can never attain such a value thatthe amplification of waves by the amplifier is actually stopped. Thedetected cur, rent will therefore be substantially constant for allintensities of the incoming wave greater than some low value dependingon the initial sensitivity of the system. -Since the detected currentremains constant, it follows that the intermediate frequency carrierwa"e.-from which it is produced must also remain constant.

The relationship between the current intensity of the intermediatefrequency carrier wave and the E. M. F. of the incoming signal carrierwave is shown by the curve of Fig. 3. The abscissae are proportional tothe E. M. F. of the incoming carrier wave measured at the inputterminals of the amplifier 3, and the ordinates are proportional to thecurrent intensity denoted by I of the intermediate frequency carrierwave at the point where it enters the band filter.

The horizontal line K, K represents the limiting value that the currentI may approach. but which it cannot equal or exceed. The ordinate GK isproportional to, the in.-

tensity that roduces the limiting value OJ in Fig. 2, o the controlcurrent I -The slope of the curve at the origin of the coordinates isproportional to the initial sensitivity of the system; namely thesensitivity that would be obtained'if the control system were madeinoperative, or the sensitivity that is obtained for such lowintensities of the incoming wave that the efi'ectof the control isnegligible. Obviously, as the initlal sensitivity of the system isincreased, the control system will be operative to effect -regulation atlower and lower intensities of the incoming waves.

The uniformity of the intermediate frequencycarrier wave resulting fromthis method of control,'corresponds to transmission through a medium ofconstant attenuation; the sound waves of the impressed message arereproduced with a strength that faithfully follows their originalintensity regardless of the variations to which radio waves are subjectin the medium.

It should be noted that the control affects only those waves that arerepeated by the controlled amplifiers in the normal manner; it is noteffective upon such portion of the waves that may be transmitteddirectly through the capacities of the amplifier electrodes.

Strong incoming waves might, under certain conditions, cause sufficientcurrent to be transmitted through the amplified capacities to cause thecontrol current I to reach and exceed the limiting value at which theamplifiers become blocked. Stronger wave intensities would then beuncontrolled, the amplifiers operating merely as fixed capacity networksto produce a fixed attenuation of the waves. By neutralizing thecoupling between the input and the output circuits of, the controlledamplifiers in the manner hereinbefore described, the direct transmissionof the incoming wave is largely prevented and the range of intensitiesthat can be controlled is greatly increased.

The use of two controlled amplifiers in tandem further increases therange that can be controlled by increasing the attenuation to whichdirectly transmitted waves are subjected. Satisfactory controlmay,however, be obtained over a wide range of ingensities using only onecontrolled ampli- An additional feature of the system described abovethat tends to increase the operating range of intensities, is thearrangement whereby the auxiliary wave from the local source issubjected to the action of the controlled amplifiers. As the intensityof the incoming wave increases and, through the action of the controlcircuit, reduces the effective amplification of the amplifiers, thestrength of the auxiliary wave E. M. F. impressed upon the detector 5 isreduced.

Hence the intermediate frequency carrier approaches the limiting value.at a slower rate than it would if the auxiliary wave were notcontrolled, and further, the component of the intermediate carrier dueto the incoming wave is very small. Both'of these factors are effectivein extending the operating range.

With this arrangement, the demodulation of strong incoming waves in thedetector 5 takes place with a considerably weakened auxiliary wave. Sucha condition may give rise to a larger number of harmonics and otherundesired modulation products, but these are suppressed by the bandfilter 6.

An alternative arrangement of the circuit may be used in which theauxiliary wave is impressed directly upon the demodulator 5 and istherefore not subject to the action of the controlled amplifiers. Topermit the use of this arrangement, a switch 28 is provided by which thelocal source 2 may be connected to a coupling transformer 30, thesecondary of which is connected in series with the input circuit of thedemodulator. With this arrangement, constant conditions in thedemodulator are obtained since the incoming wave is maintained atsubstantially constant intensity by the control current.

The response of the control to changes in the strength of the incomingwave is exceedingly rapid; that it is-not instantaneous is due only tothe presence of the condensers 22 and 20 which are connected in parallelwith the resistance 18. The condensers 22 are inserted to provide a lowimpedance conmotion for the high frequency waves between the cathodesand the tuning coils in the amplifier input circuits. The condenser 20is provided to absorb the alternating current in the output of thedetector 16. They are effective in reducing the rapidity of responsebecause they re uire a certain small time to charge or disc arge whenthe applied potential is changed. It is found that the total-capacitymay be as great as 1 mfd. without increasing the response time aboveone-fiftieth of a second.

In experimental tests with a system of the type described but havingonly one controlled amplifier, received signals have been maintainedconstant in strength within 10% while the impressed wave intensitiesvaried from a minimum to a maximum value one thousand times greater.

The operation of the invention has been described in connection-with adouble detection system, but it is obvious that the principles involvedmay be applied to the control of receiving systems in which no auxiliarywave is used and which employ only one demodulation of the incomingwaves. Instead of an intermediate frequency carrier Wave, the carriercomponent of the incoming waves would be selected to produce the controlcurrent, the selection, asin the system described, being made after, thewave has passed through the controlled amplifier. The use of the doubledetection system, however, by establishing an intermediate carrier waveof relatively low frequency, makes it possible to separate the carriermore completely from its accompanying side frequencies. and thereby tosecure more exact regulation.

What is claimed is:

1. In a wave receiving system of the type in which an incoming carrierwave is demodulated with alocally generated heterodyne wave to produce abeat wave of a different frequency, a high frequency amplifier adaptedto amplify both the incoming carrier wave and the heterodyne wave beforedemodulation, demodulating means, a rectifier adapted to produce adirect current from the beat wave, and means for controlling theefficiency of said amplifier in accordance with the strength of therectified current whereby the intensity of the heterodyne wave appliedto said demodulating means is diminished as the intensity of theincoming wave increases.

2. In a wave receiving system of the type in which an incoming carrierwave is demodulated with a locally generated heterodyne wave to producea beat wave of a different frequency, means comprising a three elec- Itrode space discharge amplifier for amplifying both the incoming waveand the heterodyne wave before demodulation, demodulating means adaptedto receive the amplified waves from said amplifier, a rectifying devicefor rectifying the beat waves produced by said demodulator and means forimpressing the potential of the rectified current upon the controlelectrode of said amplifier, whereby the resistance of the spacedischarge path is increased in accordance with the intensity of theincoming wave.

3. In a heterodyne receiving system in which a space discharge amplifieris adapted to amplify the incoming waves and the beterodyne waves beforedetection, the method of controlling the sensitivitywhich comouslyamplifying the incoming wave and the =tected signals which comprisessimultanethe heterodyne wave tectibn, and varying e degree ofamplification in an opposite sense to and in synchronism with variationsof the intensity of the 10 received wave.

In Witness whereof, I hereunto subscribe mxname this 28th day ofNovember, A. D. 19 I HARALD T. FRIIS.

